The invention relates to a process for producing hydrocarbons, in which a first feed substream and a second feed substream are obtained from a hydrocarbonaceous feed stream, of which the first feed substream is converted by means of partial oxidation or autothermal reforming to a first synthesis gas stream and the second feed substream is converted by means of steam reforming to a second synthesis gas stream and subsequently combined with the first synthesis gas stream to give a third synthesis gas stream, of which at least a first portion is converted by Fischer-Tropsch synthesis to a crude product stream comprising hydrocarbons of different chain lengths, from which light hydrocarbons are separated in a tail gas, in order to recycle them and use them in the partial oxidation or autothermal reforming.
A process of this kind is known, for example, from WO2006/117499A1. Through the parallel connection of partial oxidation (PDX) or autothermal reforming (ATR) and steam reforming, it is advantageously possible to combine the strengths of the processes used, by means of which the ratio of hydrogen to carbon monoxide in the synthesis gas can be established in a comparatively simple and inexpensive manner within a wide range of values and in accordance with the requirements of the Fischer-Tropsch synthesis. At the same time, the efficiency of the synthesis gas production is distinctly increased compared to the individual processes. Furthermore, the use of the hydrocarbons present in the tail gas in PDX or ATR contributes to a high yield of the process.
The Fischer-Tropsch synthesis typically affords a crude product stream which especially includes synthetic oil, called “synthetic crude”, light hydrocarbons having four or fewer carbon atoms, heavy hydrocarbons having five or more (preferably up to 60) carbon atoms, and unconverted synthesis gas. Typically, the tail gas is separated from the crude product stream in a cold trap and comprises, as well as saturated and unsaturated light hydrocarbons, also oxygenates and unconverted synthesis gas.
The first feed substream, after preheating with cooling process streams, is mixed with the recycled tail gas, introduced into the PDX or ATR reactor and reacted in an exothermic process with oxygen which is supplied to the reactor with technical grade purity and in a substoichiometric amount.
The recycling of the tail gas limits the maximum possible preheating temperature of the feedstocks for PDX or ATR to temperatures at which the unsaturated hydrocarbons do not yet break down and lead to soot deposits in conduits and the reactor. In order nevertheless to be able to attain the temperatures needed for the conversion of the feedstocks, an elevated amount of oxygen has to be provided, which considerably impairs the economic viability of the process.